The present invention relates to a spindle unit for machine tools that facilitates an automatic tool change, as well as to a corresponding method for operating such spindle unit.
Motor-driven milling spindles typically include a shaft with a shrink-fit rotor arranged between two bearings assemblies. A tool tensioning system consisting of a chucking head, tie rod and typically a disk spring arrangement is arranged in the interior of the shaft. Frequently, a tensioning system sensor located on the tie rod and a tool loosening rod are typically attached to the end of the spindle unit. Optionally, a rotary feedthrough that is supported in roller bearings can be coupled by way of a connecting piece.
This type of spindle with a tool chuck is disclosed in the German patent publication DE 199 37 447. A tie rod that can rotate and axially move with the spindle is arranged in the spindle for clamping and/or loosening the tool chuck. At least one contactless operating sensor is provided for measuring the positions of the tool chuck. A sensor that measures continues the displacement of the tie rod is provided to better control clamping of the tool.
The German patent publication DE 36 29 453 describes an electromechanical device for generating an axial force for operating collets. An electric motor moves a hollow tie rod via a spur gear and a spindle drive in an axial direction. To reduce the friction forces caused by operating in a relatively small installation space, the rotor is connected with a pinion which engages with a toothed gear that is secured on an spindle that is supported in a housing and is prevented from moving in an axial direction. A spindle nut, which is connected to the tie rod that is supported in the housing for axial movement relative to the housing, is arranged on the spindle. Consequently, a separate electric motor has to be used for moving the tie rod.
It may sometimes be desirable to separate an entire spindle unit into a drive unit and a coupled anterior spindle, for example for exchanging or automatically changing the anterior spindle, for being able to use larger or smaller tools, or for intermediately connecting a two-stage planetary drive to increase the torque.
With conventional spindles, the tie rod, which has to remain in the anterior spindle, disadvantageously has to extend through the coupling and the motor shaft so as to reach the tool changing assembly and the tie rod sensor located behind the motor. This can not only result in a complex configuration due to the limited space reasons, but can also cause dynamical problems associated with oscillations. Moreover, when the anterior spindle is changed, the area behind the drive motor would need to be accessed when mounting/exchanging a tool, which would negate any advantages achieved by separating these components.
It would therefore be desirable and advantageous to provide an improved spindle arrangement for machine tools and a corresponding method for operating such spindle arrangement, which obviates prior art shortcomings and is able to specifically facilitate automatic tool changes.